126 
ISLAND LIFE. 
[part i. 
however, have been very different from this, because the 
temperature of a place does not depend so much on the amount 
of heat it receives directly from the sun, as on the amount 
brought to it or carried away from it by warm or cold winds. 
We often have it bitterly cold in the middle of May when we 
are receiving as much sun heat as many parts of the tropics, 
but we get cold winds from the iceberg-laden North Atlantic, 
and this partially neutralises the effect of the sun. So we often 
have it very mild in December if south-westerly winds bring 
us warm moist air from the Gulf-stream. But though the above 
method does not give correct results for any one time or place, 
it is more nearly correct for very large areas, because all the 
sensible surface-heat which produces climates comes from the 
sun, and its proportionate amount may be very nearly calculated 
in the manner above described. We may therefore say, generally, 
that during our northern winter, at the time of the glacial epoch, 
the northern hemisphere was receiving so much less heat from 
the sun as to lower its surface temperature on an average about 
35° F., while during the height of summer of the same period 
it would be receiving so much more heat as would suffice to 
raise its mean temperature about 60° F. above what it is now. 
The winter, however, would be long and the summer short, the 
difference being twenty-six days. 
We have here certainly a superabundant amount of cold in 
winter to produce a glacial period, 1 especially as this cold would 
1 In a letter to Nature of October 30th, 1879, the Rev. 0. Fisher calls 
attention to a result arrived at by Pouillet, that the temperature which the 
surface of the ground would assume if the sun were extinguished would 
be -128°F. instead of -239°F. If this corrected amount were used in 
our calculations, the January temperature of England during the glacial 
epoch would come out 17° F., and this Mr. Fisher thinks not low enough to 
cause any extreme difference from the present climate. In this opinion, 
however, I cannot agree with him. On the contrary, it would, I think, be 
a relief to the theory were the amounts of decrease of temperature in 
winter and increase in summer rendered more moderate, since according 
to the usual calculation (which I have adopted) the differences are un- 
necessarily great. I cannot therefore think that this modification of the 
temperatures, should it be ultimately proved to be correct (which is 
altogether denied by Dr. Croll), would be any serious objection to the 
